KABUL—A Swedish journalist was gunned down in a brazen daytime attack in the heart of Kabul's high-security diplomatic district, as the country braced for a spate of insurgent activity ahead of presidential elections next month.

Swedish Radio identified the victim as Nils Horner, 51 years old, the broadcaster's Asia correspondent who had just arrived in Kabul to cover the April 5 election. Such assassinations of foreign civilians on...